Lasers are used as light sources in numerous applications. Among the most commonly employed laser sources are Fabry-Pérot (FP), distributed Bragg reflector (DBR) and distributed feedback (DFB) laser diodes formed of compound semiconductors. Typically, the laser light is emitted from a partially transparent reflector that establishes an optical resonator or cavity required for the laser operation. The partially transparent reflector is commonly formed by a cleaved facet, an etched facet, or a DBR mirror.
To put a laser into practical use, it needs to be combined with additional optical and/or optoelectronic components in an optical module. Among these components are, for example, beam-splitters required to realize output power monitors, etalons to realize wavelength lockers, or variable optical attenuators (VOAs) required for shuttering or leveling of the output power.
These components and assembly processes related to them generate a considerable cost (typically more than 80% of a total production cost), and give rise to additional optical loss that reduces the output power of a laser module. Therefore, it is highly desirable to eliminate, such external components in order to reduce the production cost and to enhance the optical module performance.
Many of the above-mentioned functionalities provided by external bulk optical/optoelectronic components can be realized in various material systems as monolithic waveguide-based devices that can be produced on a chip-level scale and, thus, at reduced cost and in many cases enhanced performance. Examples for such monolithic waveguide-based devices are silicon- or silica-based planar lightwave circuits (PLCs) as well as the multitude of compound semiconductor-based optoelectronic devices.
The coupling of the laser output, which is usually emitted from the laser facet into a free space, into a second chip provides difficulty in practice due to an associated high loss.
Here, a brief overview of currently available outcoupling and integration schemes as well as related technologies will be given below.
At first, a Fabry-Pérot (FP) laser will be described. In case of a FP laser, a laser cavity is usually formed by cleaved facets (eventually having a low- or high-reflection (LR or HR) coating) that act as mirrors. To integrate such a device for example with power monitors, a wavelength locker, or a variable optical attenuator, either of a large number of external bulk optical/optoelectronic components would be employed as shown in FIG. 1) or a lens would have to be used to couple the light emitted from the laser facet to a second chip that provides a desired function (FIG. 2). While the former approach requires complex (i.e. high-precision) mounting techniques and expensive bulk components and additionally results in a large coupling loss, the latter approach avoids expensive bulk components but also suffers from a relatively large coupling loss and the need for high-precision mounting.
Next, a DBR laser will be described with reference to FIG. 3.
In the DBR laser, a mirror facet is not required since diffraction gratings incorporated in a laser diode provide reflection to form a laser cavity. Consequently, a laser light output in such devices can in principle be coupled directly to a wave guide in principle, and therefore, an additional optical function can be comparatively easily integrated. However, as the output light is emitted through a lossy diffraction grating, the DBR laser integration concept is impaired by a considerable output power penalty. Moreover, limitations arise from the fact that it is difficult, impractical, or impossible to realize certain reflection spectrum shapes, such as reflection spectrum that is substantially flat and has a certain uniform reflectivity over a wide wavelength range (for example, 100 nm or more, as required for some applications).
Also, integration using monolithically integrated wideband mirrors is known.
Other outcoupling schemes for the FP/DBR laser that strive to provide a possibility to integrate further functional units with the laser, are based on gap mirrors (FIG. 4) (a first related art), very high index contrast gratings (a second related art), and MMI-based mirrors (a third related art). The first related art is “Integration of Functional SOA on the Gain Chip of an External Cavity Wavelength Tunable Laser Using Etched Mirror Technology” (IEEE J. Select. Topics Quantum Electron., vol. 13, no. 5, pp. 1104-1111, 2007) by M. L. Nielsen et. al. Also, the second related art is U.S. Pat. No. 6,022,980 B2 titled “Tunable Semiconductor Laser with Integrated Wideband Grating”, and granted to L. P. O. Lundqvist on 23 Nov. 2004. Also, the third related art is international. Publication (WO 2006/104441 A1) of an international patent application by P.-J. Rigole at al., titled “Integrated Photonic Circuit”, and filed on 5 Oct. 2006.
The former two schemes are technologically demanding, and in particular, the gap mirror approach results in a very large coupling loss. The latter approach based on an MMI reflector is only suitable for a reflectivity of 50% and, thus, its applicability is very limited.
Also, the FP/DBR laser with an intracavity tap for power monitor is known. In a fourth related art (“A 1.3-μm Wavelength Laser with an Integrated Output Power Monitor Using a Directional Coupler Optical Power Tap”, (IEEE Photon. Technol. Lett., vol. 8, no. 3, pp. 384-366, 1996) by U. Koren et al.), a FP laser diode, is presented that uses an intracavity tap to guide a small fraction of light to an integrated monitor photodiode, as shown in FIG. 5. This tap is, however, not used to outcouple the main light output from the laser (which is still emitted from the cleaved front facet of the laser chip) but only a small fraction of the total power that is needed for the power monitoring purpose. In the fourth related art, there is no comment on the feasibility to enhance the output power performance of certain FP/DBR laser diode configurations by outcoupling the in light output from the laser via an intracavity tap.
Also, a ring laser with intracavity outcoupling tap is known. In a ring laser, the light is traveling in a closed loop and, therefore, the only way to remove light from this closed loop is via an intracavity tap and is, hence, a very common outcoupling scheme in ring lasers, as disclosed in fifth and sixth related arts. The fifth related art is “Impact of Output Coupler Configuration on Operating Characteristics of Semiconductor Ring Lasers”, (J. Lightwave Technol., vol. 13, no. 7, pp. 1500-1507, 1995) by T. F. Krauss et al. The sixth related art is “Stable and Fast Wavelength Switching in Digitally Tunable Laser Using Chirped Ladder Filter”, (IEEE J. Select. Topics Quantum Electron., vol. 13, No. 5, pp. 1122-1128, 2007) by S. Matsuo at al.